HLA-DRB1 and rheumatoid arthritis: Although no association was found between SE and anti-P. gingivalis antibodies; according to the de Vries’s classification, an association existed between HLA-DRB1 neutral alleles, and high titres of IgG anti-P.gingivalis antibodies for RA, focusing on novel associations between P.gingivalis and RA. NETosis or intracellular infections by gut pathobionts may also contribute to citrullination. This could explain why anti-Pg antibodies were not clearly associated with SE or ACPAs in patients with RA.